Elements such as sulfur, phosphorous, silicon, and oxygen have been found to be undesirable elements which are always present in iron, copper and other metals. The presence of such elements are derived primarily from the ore, the scrap and the fluxes making up the charge, and from the fuel used. For example, there presently is a great demand in the iron and steel industry for products having relatively low sulfur content, and accordingly, the removal of this element has become of paramount importance.
In terms of other elements, the removal of phosphorous from hot metal or foundry iron is critical, since it has been found that low phosphorous content improves steel and iron castings' mechanical properties, such as toughness. The removal of silicon from blast furnace liquid metal is important, since low silicon content is required for efficient dephosphorization and also for decreasing BOF slag volume and flux consumption, thereby yielding a better BOF metallic yield and better refractory performance. The removal of oxygen from liquid metals is necessary, since a low oxygen condition is required to insure integrity of cast metals. The removal of oxygen is also required in processing liquid iron and steel not only for the purpose of increasing efficiency of desulfurization but also for improving steelmaking alloying element yield and nonmetallic inclusion control for improved mechanical and surface properties of finished steel. Finally, with respect to copper melts, the removal of oxygen is critical in improving mechanical properties such as brittleness and for better electrical conductivity.
Agents utilized to remove these impurities are normally introduced into the molten metal in the form of a composition containing the agent utilized for treating a molten metal to remove unwanted impurities in admixture with other components which are added for such purposes as increasing the flowability of the composition, promoting the distribution of the agent in the melt, and generally improving the effect of such agents to remove the unwanted impurity.
The problems associated with the underutilization of such agents for removing impurities from a molten metal result in a lack of uniformity of efficiency due at least in part to difficulties in uniformly contacting the agent with the molten metal. It has been found that there is an incomplete use of the agent in that the agent is apt to pass through the melt partially unreacted.
For example, calcium carbide has the capability of combining readily with the sulfur present in molten metals. However, the use of calcium carbide presents several difficulties, particularly since calcium carbide has a specific gravity of approximately 2.4, whereas iron has a specific gravity of 7. Therefore, the calcium carbide tends to become buoyant in the molten metal and thereby decreases the time the calcium carbide is suspended in the molten metal for the purposes of reacting with the sulfur therein. Furthermore, calcium carbide does not melt at the temperatures of molten iron and steel. Accordingly, the reaction must be effected between a solid reagent and a liquid molten metal. The reaction then depends upon the direct or intimate contact between the solid calcium carbide and the molten metal and therefore, the calcium carbide particle separation and particle penetration across the gas/metal interface into the molten metal itself.
To increase the penetration into the melt of agents used in removing impurities from a molten metal and thereby attempt to increase the dwell time and maximum surface contact between the agents and the metal, several methods have been suggested, such as increased stirring of the agent in the metal, plunging the agents--for example, magnesium impregnated coke--under the surface of the molten metal, or injecting under pressure particulate desulfurizing agents-for example, lime, calcium carbide, or calcium silicide--into the metal beneath the surface. Injected agents may be admixed with gas release compounds such as alkaline-earth carbonates, diamide lime (a precipitated carbon-containing calcium carbonate formed as a byproduct from the manufacture of dicyandiamide), which decompose to release a gas under the temperature conditions of the molten metal to achieve better mixing of the agent with the molten metal through agitation.
However, calcium carbide, for example, is poorly wetted by high carbon-containing iron. Poorly wetted desulfurizing agents in gas or mechanically stirred iron tend to resist penetration beneath the melt surface due to the high interfacial tension between the solid particles and the melt or melt/air interface. In gas injection systems where gas bubbles may be present from reagent carrier gas or from the heating of gas generating stirring agents, such as alkaline earth carbonates, the high melt surface tension repels the solid particles at the gas/molten metal interface so that a large fraction of the injected particles are carried to the melt surface inside gas bubbles without reacting with the sulfur contained in the molten metal. The degree of wettability between solid agents used to remove impurities from a molten metal and the molten metal incorporates the concept of interfacial tension between a solid in contact with a liquid or liquid and gas interface.
There is disclosed another method for improving the efficiency of an agent to remove an impurity from a molten metal in U.S. Pat. No. 3,885,956 wherein calcium carbide particles are coated with magnesium for the purpose of protecting the calcium carbide from exposure to the atmosphere which thereby prevents the reaction of calcium carbide to form acetylene prior to its introduction into the melt. However, this coating does not increase the ability of the agent to penetrate the gas/liquid interface.
Another instance of coating an agent is shown when utilizing magnesium as a desulfurizing agent, where it has been found with respect to desulfurization with magnesium that magnesium or magnesium-based desulfurizing agents tend to "flash" or vaporize when added to the molten metal due to the fact that the magnesium metal has a boiling point less than that of a molten metal, such as iron or steel. Accordingly, the vaporization of the magnesium causes the magnesium to rise through the molten metal without fully reacting with the sulfur. This thereby decreases dwell time and limits the efficiency of the magnesium as a desulfurizing agent. To overcome this problem, there is disclosed in Japanese Abstract No. 136,199 a method of coating magnesium with zirconium oxide and titanium oxide to insulate the magnesium, thereby reducing its vaporization rate in the molten bath and causing it to have a longer dwell time in the bath to react with the sulfur contained therein.
Despite these various suggested improvements, the effectiveness and efficiency of a desulfurizing agent or its method of application still leaves a great deal to be desired. Accordingly, the industry has utilized a greater amount of agent to remove impurities from a molten metal at great expense to achieve the desired results.